netdev - [PATCH net] inet_ecn: Fix endianness of checksum update when setting ECT(1)

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Message-Id: <20201130183705.17540-1-toke@redhat.com>
Date:   Mon, 30 Nov 2020 19:37:05 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        netdev@...r.kernel.org, Jonathan Morton <chromatix99@...il.com>,
        Pete Heist <pete@...stp.net>
Subject: [PATCH net] inet_ecn: Fix endianness of checksum update when setting ECT(1)

When adding support for propagating ECT(1) marking in IP headers it seems I
suffered from endianness-confusion in the checksum update calculation: In
fact the ECN field is in the *lower* bits of the first 16-bit word of the
IP header when calculating in network byte order. This means that the
addition performed to update the checksum field was wrong; let's fix that.

Fixes: b723748750ec ("tunnel: Propagate ECT(1) when decapsulating as recommended by RFC6040")
Reported-by: Jonathan Morton <chromatix99@...il.com>
Tested-by: Pete Heist <pete@...stp.net>
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 include/net/inet_ecn.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/inet_ecn.h b/include/net/inet_ecn.h
index e1eaf1780288..563457fec557 100644
--- a/include/net/inet_ecn.h
+++ b/include/net/inet_ecn.h
@@ -107,7 +107,7 @@ static inline int IP_ECN_set_ect1(struct iphdr *iph)
 	if ((iph->tos & INET_ECN_MASK) != INET_ECN_ECT_0)
 		return 0;
 
-	check += (__force u16)htons(0x100);
+	check += (__force u16)htons(0x1);
 
 	iph->check = (__force __sum16)(check + (check>=0xFFFF));
 	iph->tos ^= INET_ECN_MASK;
-- 
2.29.2